Conductive polymers are increasingly gaining in economic importance, since polymers offer advantages over metals with regard to processability, weight and the selective adjustment of properties by means of chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene vinylenes). Layers of conductive polymers are widely used technically, for example as polymeric counter-electrodes in capacitors or for through-hole plating in printed circuit boards. Conductive polymers are produced by chemical or electrochemical oxidation from monomeric precursors, such as for example from optionally substituted thiophenes, pyrroles and anilines and optionally oligomeric derivatives thereof. Chemical oxidative polymerization in particular is widespread, since it is technically simple to carry out in a liquid medium or on diverse substrates.
A particularly important polythiophene used technically is poly(ethylene-3,4-dioxythiophene) (PEDOT or PEDT) as described for example in EP 0 339 340 A2, which is produced by chemical polymerization of ethylene-3,4-dioxythiophene (EDOT or EDT) and which in its Oxidized form exhibits very high conductivity values. A survey of numerous poly(alkylene-3,4-dioxythiophene) derivatives, in particular poly(ethylene-3,4-dioxythiophene) derivatives, their monomer units, syntheses and applications is provided by L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12, (2000) p. 481-494.
Dispersions of PEDOT with polyanions, such as for example polystyrene sulphonic acid (PSS), as disclosed for example in EP 0 440 957 A2, have acquired particular importance in industry. These dispersions can be used to produce transparent, conductive films which have found numerous applications, for example as an antistatic coating or as a hole-injection layer in organic light-emitting diodes (OLEDs), as shown in EP 1 227 529 A2.
The polymerization of EDOT takes place in an aqueous solution of the polyanion to form a polyelectrolyte complex. Cationic polythiophenes, which for the purposes of charge compensation comprise polymeric anions as counterions, are often also referred to by experts as polythiophene/polyanion complexes. By virtue of the polyelectrolyte properties of PEDOT as a polycation and PSS as a polyanion this complex is not a true solution but rather a dispersion. The extent to which polymers or portions of polymers are dissolved or dispersed is dependent on the mass ratio of the polycation and the polyanion, on the charge density of the polymers, on the salt concentration of the environment and on the nature of the surrounding medium (V. Kabanov, Russian Chemical Reviews 74, 2005, 3-20). The transitions here can be fluid. For that reason no distinction is made hereafter between the terms “dispersed” and “dissolved”. Likewise no distinction is made between “dispersion” and “solution” or between “dispersant” and “solvent”. Rather these terms are used synonymously hereafter.
As described above, complexes of PEDOT and PSS have found a wide variety of applications. However, the disadvantage of using PSS as the polyanion is that it is soluble in water and in water-miscible organic solvents, for instance in lower alcohols such as ethanol or methanol, but not in water-immiscible organic solvents. The dispersing of conductive polymers such as PEDOT in water-immiscible solvents is however desirable in some cases, since firstly such solvents can be removed comparatively easily by evaporation and such solvent systems are distinguished by particularly good film-forming properties. Secondly, dispersions comprising PEDOT are frequently used in combination with paint systems, which however are often based on water-immiscible solvents or solvent systems.
Moreover, aqueous PEDOT/PSS dispersions have the disadvantage that the lifetime of OLEDs with hole-injection layers produced from these dispersions is in need of further improvement. In particular, when PEDOT/PSS dispersions are used to produce hole-injection layers in OLEDs, the luminance of the OLEDs decreases comparatively quickly in some circumstances.
PEDOT-comprising systems based on non-aqueous or low-water-content solvent systems are already known from the prior art.
Thus EP-A-1 373 356 and WO-A-2003/048228 for instance describe the production of polythiophene-polyanion complexes in anhydrous or low-water-content solvents. The solvent water is exchanged here for another water-miscible organic solvent. To this end the second solvent is added and then water is removed, by distillation for example. The disadvantage of this procedure is that the distillation requires a two-stage process to be used. The added solvent must moreover be miscible with water, and this likewise limits the choice to polar solvents.
In JP-A-2005-068166 Otani et al. describe a method in which a conductive polymer such as PEDOT is first dried and then dispersed in an organic solvent. Organic solvents having a dielectric constant of 5 or more are cited in particular. Isopropyl alcohol and gamma-butyrolactone are cited in the examples. This method too has the disadvantage that polar solvents are necessary for the renewed dissolution. This method is also disadvantageous in that the conductive polymer has to be synthesised first, then dried and then dispersed again. Otani et al. also disclose no polythiophene-polyanion complexes.
WO-A-2009/135752 likewise describes PEDOT-comprising compositions based on water-immiscible solvent systems. As polyanions the PEDOT/polyanion complexes described in this prior art preferably comprise styrene-styrene sulphonic acid copolymers in which the styrene units are alkylated in para-position, such as for example poly(p-styrene ethyl sulphonate-co-p-dodecylstyrene). The disadvantage of this approach, however, is that the production of such styrene-styrene sulphonic acid copolymers requires a comparatively large number of synthesis steps. In particular such polyanions are very difficult to obtain commercially. In addition, the conductivity of these layers described in this prior art is comparatively low.
A number of works describe furthermore how a polythiophene can be made soluble by attaching side groups to the thiophene monomer and then polymerizing it or by producing a block copolymer from thiophene units and units intended to increase the solubility. Thus Luebben et al. (Polymeric Materials: Science & Engineering 2004, 91, 979) describe the production of a block copolymer from PEDOT and polyethylene glycol. Perchlorate and p-toluenesulphonic acid act as counterions here. The polymers are soluble in polar organic solvents such as propylene carbonate and nitromethane. Conductivities in a range from 10−4 S/cm to 1 S/cm are measured. The cited block copolymers have the disadvantage, however, that they are soluble only in very polar solvents. Moreover, the selected counterions make no contribution to film formation, so conductive films cannot be formed with these block copolymers. Further publications describe the production of organic polythiophene solutions by introducing side groups to the thiophene which contribute to the solubility. Thus Yamamoto et al. (Polymer 43, 2002, 711-719) describe the production of hexyl derivatives of PEDOT which as a neutral molecule is soluble in organic solvents. In principle this method has the disadvantage that the introduction of side chains to the thiophene influences not only the solubility properties but also the electronic properties of the molecule.